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Subject: Re: [PATCH V2] mm, page_alloc: fix core hung in free_pcppages_bulk()
To:     David Hildenbrand <david@redhat.com>, akpm@linux-foundation.org,
        mhocko@suse.com, vbabka@suse.cz, rientjes@google.com,
        linux-mm@kvack.org
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References: <1597150703-19003-1-git-send-email-charante@codeaurora.org>
 <fdf574c8-82be-6bde-b73b-c97055f530a8@redhat.com>
 <848b7d60-2995-d9ae-0055-f3864dece11f@codeaurora.org>
 <db7ac714-f508-85a4-1af8-9733e680dc69@redhat.com>
From:   Charan Teja Kalla <charante@codeaurora.org>
Message-ID: <2a2ec65a-ad93-7adc-76a0-246b9b328155@codeaurora.org>
Date:   Wed, 12 Aug 2020 15:41:30 +0530
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On 8/12/2020 3:30 PM, David Hildenbrand wrote:
> On 12.08.20 11:46, Charan Teja Kalla wrote:
>>
>> Thanks David for the inputs.
>>
>> On 8/12/2020 2:35 AM, David Hildenbrand wrote:
>>> On 11.08.20 14:58, Charan Teja Reddy wrote:
>>>> The following race is observed with the repeated online, offline and a
>>>> delay between two successive online of memory blocks of movable zone.
>>>>
>>>> P1						P2
>>>>
>>>> Online the first memory block in
>>>> the movable zone. The pcp struct
>>>> values are initialized to default
>>>> values,i.e., pcp->high = 0 &
>>>> pcp->batch = 1.
>>>>
>>>> 					Allocate the pages from the
>>>> 					movable zone.
>>>>
>>>> Try to Online the second memory
>>>> block in the movable zone thus it
>>>> entered the online_pages() but yet
>>>> to call zone_pcp_update().
>>>> 					This process is entered into
>>>> 					the exit path thus it tries
>>>> 					to release the order-0 pages
>>>> 					to pcp lists through
>>>> 					free_unref_page_commit().
>>>> 					As pcp->high = 0, pcp->count = 1
>>>> 					proceed to call the function
>>>> 					free_pcppages_bulk().
>>>> Update the pcp values thus the
>>>> new pcp values are like, say,
>>>> pcp->high = 378, pcp->batch = 63.
>>>> 					Read the pcp's batch value using
>>>> 					READ_ONCE() and pass the same to
>>>> 					free_pcppages_bulk(), pcp values
>>>> 					passed here are, batch = 63,
>>>> 					count = 1.
>>>>
>>>> 					Since num of pages in the pcp
>>>> 					lists are less than ->batch,
>>>> 					then it will stuck in
>>>> 					while(list_empty(list)) loop
>>>> 					with interrupts disabled thus
>>>> 					a core hung.
>>>>
>>>> Avoid this by ensuring free_pcppages_bulk() is called with proper count
>>>> of pcp list pages.
>>>>
>>>> The mentioned race is some what easily reproducible without [1] because
>>>> pcp's are not updated for the first memory block online and thus there
>>>> is a enough race window for P2 between alloc+free and pcp struct values
>>>> update through onlining of second memory block.
>>>>
>>>> With [1], the race is still exists but it is very much narrow as we
>>>> update the pcp struct values for the first memory block online itself.
>>>>
>>>> [1]: https://patchwork.kernel.org/patch/11696389/
>>>>
>>>
>>> IIUC, this is not limited to the movable zone, it could also happen in
>>> corner cases with the normal zone (e.g., hotplug to a node that only has
>>> DMA memory, or no other memory yet).
>>
>> Yes, this is my understanding too. I explained the above race in terms
>> of just movable zone for which it is observed. We can add the below line
>> in the end in patch commit message:
>> "This is not limited to the movable zone, it could also happen in cases
>> with the normal zone (e.g., hotplug to a node that only has DMA memory,
>> or no other memory yet)."
> 
> Yeah, that makes sense!
> 
>>
>> Just curious, there exists such systems where just a dma zone present
>> and we hot add the normal zone? I am not aware such thing in the
>> embedded world.
> 
> You can easily create such setups using QEMU.
> 
> IIRC, just specify a QEMU guest with 2G initial memory and a single NUMA
> node, or 4G initial memory and two NUMA nodes. Then hotplug memory.
> 
> (IIRC kata containers always start a VM with 2G and then hotplug memory)
>
I see. Thanks for letting me know this.

>>>
>>>> Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
>>>> ---
>>>>
>>>> v1: https://patchwork.kernel.org/patch/11707637/
>>>>
>>>>  mm/page_alloc.c | 5 +++++
>>>>  1 file changed, 5 insertions(+)
>>>>
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index e4896e6..839039f 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -1304,6 +1304,11 @@ static void free_pcppages_bulk(struct zone *zone, int count,
>>>>  	struct page *page, *tmp;
>>>>  	LIST_HEAD(head);
>>>>  
>>>> +	/*
>>>> +	 * Ensure proper count is passed which otherwise would stuck in the
>>>> +	 * below while (list_empty(list)) loop.
>>>> +	 */
>>>> +	count = min(pcp->count, count);
>>>>  	while (count) {
>>>>  		struct list_head *list;
>>>>  
>>>>
>>>
>>> Fixes: and Cc: stable... tags?
>>
>> Fixes: 5f8dcc21211a ("page-allocator: split per-cpu list into
>> one-list-per-migrate-type")
>> Cc: <stable@vger.kernel.org> [2.6+]
> 
> Did we have memory hotplug support then already?

Yes, it exist.
https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/mm/memory_hotplug.c?h=v2.6.39

> 
>>
>> I am not sure If I should have to raise V3 including these?
> 
> 
> Maybe Andrew can fixup when applying.

Okay, let Andrew decide on this. Meanwhile If you find that this patch
looks correct, ACK from you helps here.

> 
> 

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